Hydroxy-containing ethylenically unsaturated polyesters prepared from glycidyl polyethers of polyhydric phenols and ethylenically unsaturated monocarboxylic acids tend toward premature gelation. In other words, the polyesters, if stored for long periods of time before use, will increase in viscosity (gel) to a value which severely limits their use for many applications where low viscosity is important.
Such premature gelation is significantly reduced by the addition of a dialkylhydroxylamine. See, for example, U.S. Pat. No. 3,408,422, issued Oct. 29, 1968.
When, however, the epoxy compound contains tertiary, allylic or benzylic hydrogens, such as the so-called saturated epoxy resins, there appears to be a propensity toward auto-oxidation and peroxide formation. Accordingly, when such epoxy compounds are esterfied with ethylenically unsaturated acids, the reaction mixture gels during the esterification step. It has now been found that if the saturated epoxy resin is pretreated with a dialkylhydroxylamine before the esterification, the esterification can be effected without premature gelation to produce unsaturated vinyl esters exhibiting excellent stability.